1. Field of the Invention
The present invention pertains to an intrinsically safe battery circuit, and more particularly, to an intrinsically safe battery hybrid circuit which prevents arcing and allows full current from the battery during operation.
2. Description of the Prior Art
An intrinsically safe battery is defined as a cell pack whose energy is limited to a point so as not to cause ignition as properly used in various atmospheres. The atmospheres considered dangerous are divided into several groups, and the groups are determined by the amount of energy required to cause ignition.
A regulatory agency most recognized in the United States is Factory Mutual ("FM"). FM in an insurance company specializing in hazardous applications. All testing is dictated and conducted on site at FM. Any damage or injury occurred in an accident involving a battery or other equipment not "FM Approved" voids Factory Mutual's liability. For this reason, customers insist on "FM Approved" batteries which are intrinsically safe.
Testing involves a variety of dielectric and physical construction checks. These checks are intended to insure the stability of the components used or limit the energy from the cells. Electrical testing is conducted on a spark generator.
A chamber is constructed which is air tight and includes a viewing window. Inside, a random spaced circular set of contact pins, resembling nails, are attached to one terminal of the battery. The other terminal attaches to a rotating arm whose tip contacts the contact pins. This essentially makes and breaks the circuit to the battery under test. Arcs are created inside the chamber, and these arcs are representative of the energy available from the battery. The different atmospheres are then pumped into the chamber and checked for ignition.
In the prior art, the most common method of making a battery intrinsically safe has been to simply add enough series resistance to limit the energy to a safe level. This has worked well except for higher voltage cell packs such as 12 volts and over. These batteries of a larger voltage require a large value resistance at a high wattage that is physically difficult, and not acceptable to locate internally in the battery.
The present invention overcomes the disadvantages of the prior art by providing a hybrid circuit to prevent arcing, which passes all electrical tests and allows full current from the battery.